1. Field of the Invention
The present invention relates to an optical waveguide device used, e.g., in the fields of optical information processing and optical application measurement and control, and to a coherent light source and an optical apparatus using the same.
2. Description of the Related Art
Optical information recording/reproducing apparatuses can achieve higher density by using a shorter-wavelength light source. For example, a widespread compact disk (CD) apparatus uses near-infrared light having a wavelength of 780 nm, while a digital versatile disk (DVD) apparatus that can reproduce information with higher density uses a red semiconductor laser having a wavelength of 650 nm. To achieve a next-generation optical disk apparatus with even higher density, a blue laser source with even shorter wavelength has been under active development. For example, to provide a small and stable blue laser source, a second harmonic generation (hereinafter, referred to as xe2x80x9cSHGxe2x80x9d) device has been developed by using a nonlinear optical material.
FIG. 6 is a schematic view showing an example of the configuration of an optical apparatus that includes a SHG blue light source using a SHG device.
First, the SHG blue light source will be described by referring to FIG. 6.
As shown in FIG. 6, a SHG blue light source 101 includes a SHG device 103 and a semiconductor laser 104. The semiconductor laser 104 is connected directly to the SHG device 103.
The SHG device (optical waveguide device) 103 includes an optical material substrate 105. A high refractive index region with a width of about 3 xcexcm and a depth of about 2 xcexcm is formed on the optical material substrate 105 by a proton-exchange method. This high refractive index region functions as an optical waveguide 106. Infrared light having a wavelength of 850 nm is emitted from the semiconductor laser 104, focused on an entrance end face 106a of the optical waveguide 106 on the SHG device 13, and propagates in the optical waveguide 106 so as to be a fundamental guided wave. LiNbO3 crystals, which are used as a substrate material for the optical material substrate 105, have a large nonlinear optical constant. Therefore, a harmonic guided wave having half the wavelength of the fundamental light (425 nm) is excited from the electric field of the fundamental light. To compensate for a difference in propagation constant between the fundamental light and the harmonic light, a periodic polarization inversion region 107 is formed on the optical waveguide 106. The harmonic light that is excited over the entire region of the optical waveguide 106 is added coherently, which then exits from an exit end face 106 of the optical waveguide 106.
It is necessary to maintain the wavelength of the fundamental light precisely constant to ensure accurate compensation for the difference in propagation constant between the fundamental light and the harmonic light. Therefore, a distributed Bragg reflection (hereinafter, referred to as xe2x80x9cDBRxe2x80x9d) semiconductor laser is used as the semiconductor laser 104. The DBR semiconductor laser includes a DBR region and shows extremely small wavelength variations with respect to temperature or the like.
Next, the operation of an optical pickup system that includes the SHG blue light source using the SHG device will be described by referring to FIG. 6.
As shown in FIG. 6, an optical apparatus 102 includes the SHG blue light source (coherent light source) 101, a focusing optical system, and a photodetector 112. The SHG blue light source 101 includes the SHG device 103 and the semiconductor laser 104. The focusing optical system includes a collimator lens 108, polarizing beam splitter 109, a quarter-wave plate 110, and an objective lens 111.
The harmonic blue light emitted from the SHG device 103 passes through the collimator lens 108, the polarizing beam splitter 109, the quarter-wave plate 110, and the objective lens 111 in sequence, and thus is focused on an optical disk 113. The light modulated by the optical disk 113 is reflected from the polarizing beam splitter 109 and directed to the photodetector 112 through a focusing lens (not shown), thereby providing a reproduction signal. At this time, linearly polarized light emitted from the SHG device 103 in the direction parallel to the sheet of the drawing is polarized in the direction perpendicular thereto by passing through and returning to the quarter-wave plate 110. All the reflected light from the optical disk 113 is deflected by the polarizing beam splitter 109 and does not return to the side of the SHG blue light source 101.
However, the base material for the actual optical disk 113 has a birefringent property. Thus, undesired polarized components generated in the optical disk 113 may pass through the polarizing beam splitter 109 and return to the side of the SHG blue light source 101, which is referred to as return light. During reproduction of the optical disk 113, the position of the objective lens 111 is controlled so as to ensure precise focusing on the optical disk 113. Therefore, the exit end face 106b and the optical disk 113 constitute a confocal optical system, and the reflected light from the optical disk 113 is focused precisely on the exit end face 106b. 
When the reflected light from the optical disk 113 returns to the side of the SHG blue light source 101 as described above, noise is caused. To avoid this noise, various techniques have been proposed. Examples of such techniques include a method for generating a plurality of longitudinal modes by modulating a semiconductor laser with a high frequency signal and a method for also generating a plurality of longitudinal modes by causing self-oscillation in a semiconductor laser. In the field of optical communication, an optical isolator that has a magneto-optical effect generally is located between a semiconductor laser and an optical fiber so that light from the semiconductor laser is focused on the optical fiber. Moreover, another method has been proposed that prevents reflected light from returning to a semiconductor laser by cutting the entrance end face of an optical fiber or an optical waveguide so as to reflect the reflected light obliquely (JP 5(1993)-323404 A or the like).
These techniques reduce noise caused by light returning to the semiconductor laser. As a result of experiments on reproduction of the optical pickup that includes the optical waveguide type SHG device 103 shown in FIG. 6, the present inventors found noise caused by a different mechanism from that of the conventional noise induced by return light. This noise is interference noise generated when the return light focused on the exit end face 106b is reflected and interferes with light emitted from the optical waveguide 106. The output power of the SHG blue light source 101 appears to change due to this interference effect when observed from the optical disk side, and a reproduction signal of the optical disk 113 is modulated by low frequency noise, which leads to degradation of the reproduction signal. The noise induced by the return light in the semiconductor laser 104 is generated by the interaction between light inside the semiconductor laser 104 and the return light. On the other hand, the interference noise is generated by the interference between light emitted from the SHG blue light source 101 and the return light.
As described above, there are two different types of noise in the optical system that uses the optical waveguide device (the SHG device 103): low frequency interference noise and mode hopping noise. The low frequency interference noise occurs when light emitted from the SHG blue light source 101 is reflected and returns to the exit end face of the SHG blue light source 101 to cause interference in the optical system outside the SHG blue light source. The mode hopping noise results from the inside of the semiconductor laser 104. Various techniques have been proposed as a method for reducing the mode hopping noise. JP 2000-171653 A discloses a technique for reducing return light to the SHG blue light source 101 and interference noise in the confocal optical system. According to this technique, the exit end face 106b of the optical waveguide 106 tilts with respect to the direction of an optical axis passing through the optical waveguide 106 (i.e., the propagation direction of a guided wave), as shown in FIGS. 7, 8 and 9. Therefore, the harmonic light reflected from the exit end face 106b does not travel in the direction of the optical axis of the optical waveguide 106. Thus, the interference between light exiting from the SHG device 103 and the reflected light can be reduced to prevent the occurrence of interference noise.
To make the SHG blue light source 101 smaller, the semiconductor laser 104 and the optical waveguide 106 are coupled directly with high efficiency. For this purpose, the distance between the semiconductor laser 104 and the entrance end face of the SHG device 103 should be a few micrometers or less, and the optical waveguide 106 should be formed so that the direction of the optical axis of the optical waveguide 106 is substantially perpendicular to the entrance end face 106a. Moreover, to achieve high-efficiency wavelength conversion, the conditions of single phase matching should be satisfied over a long distance. Therefore, it is desirable that both the propagation direction and the propagation constant are uniform over the longest possible distance of the optical waveguide. Thus, a straight waveguide is suitable for high-efficiency wavelength conversion.
JP 5(1993)-323401 A discloses an optical wavelength conversion device that has a periodic polarization inversion structure and includes a curved optical waveguide, though the object and the effect differs from those of the present invention. The invention disclosed in JP 5(1993)-323401 A employs a curved optical waveguide that can change the propagation direction of a guided wave gradually, thereby changing the phase matching conditions of the optical waveguide in accordance with the propagation direction and increasing the tolerance of a phase-matched wavelength.
However, there is a serious problem in mass production of the SHG device 103 that includes the optical waveguide 106 whose entrance and exit end faces 106a, 106b are not parallel to each other. As shown in FIG. 10, the SHG device 103 is produced generally by optically polishing the optical material substrate 114 and then cutting the substrate into a small size. Usually, to simplify the optical polishing process and improve polishing accuracy, a relatively large optical material substrate is polished optically and then cut. When this method is used to mass-produce the SHG device 103 in which the entrance end face 106a is not parallel to the exit end face 106b, the device length varies from one device to another so that the mass production of uniform SHG devices 103 is impossible.
The conventional optical wavelength conversion device that has the periodic polarization inversion structure and includes the curved optical waveguide differs from the present invention in object and effect. Though the use of the curved optical waveguide can increase the tolerance of a phase-matched wavelength, it involves a significant reduction in the efficiency of wavelength conversion. Moreover, JP 5(1993)-323401 A fails to disclose the relationship between the entrance end face and the exit end face of the optical waveguide. It also fails to describe any problem in terms of simplicity of a mass production process.
Therefore, with the foregoing in mind, it is an object of the present invention to provide an optical waveguide device that can be mass-produced easily. It is another object of the present invention to provide a coherent light source that uses the optical waveguide device to satisfy the light source characteristics of low noise. It is yet another object of the present invention to provide an optical apparatus that uses the coherent light source to reduce interference noise caused outside of the light source.
To achieve the above objects, an optical waveguide device according to a configuration of the present invention includes a substrate provided with an optical waveguide, and an entrance end face and an exit end face formed on the end portions of the optical waveguide. The entrance end face is substantially parallel to the exit end face. The angle xcex8 between the exit end face and the direction of an optical axis of the optical waveguide at the exit end face is not 90xc2x0.
In the optical waveguide device of the present invention, it is preferable that the optical waveguide device is substantially in the form of a rectangular parallelepiped.
In the optical waveguide device of the present invention, it is preferable that the angle xcex8 satisfies xcex8xe2x89xa687xc2x0 or xcex8xe2x89xa793xc2x0.
In the optical waveguide device of the present invention, it is preferable that the angle xcex8 satisfies 80xc2x0xe2x89xa6xcex8xe2x89xa687xc2x0 or 100xc2x0xe2x89xa7xcex8xe2x89xa793xc2x0.
In the optical waveguide device of the present invention, it is preferable that the optical waveguide includes a straight waveguide that extends from the entrance end face in the direction substantially perpendicular to the entrance end face and at least one inclined waveguide that is formed between the entrance end face and the exit end face. In this case, it is preferable that a plurality of guided waves with different wavelengths propagate in the optical waveguide, and the radiation loss in the inclined waveguide differs depending on the guided waves. Moreover, it is preferable that the inclined waveguide includes a straight portion and a curved portion.
In the optical waveguide device of the present invention, it is preferable that the substrate is made of MgO-doped LiNbO3 crystals, and the entrance end face and the exit end face are substantially parallel to an X-plane or Y-plane of the crystals.
In the optical waveguide device of the present invention, it is preferable that the optical waveguide has a periodic polarization inversion structure.
A coherent light source according to a configuration of the present invention includes a semiconductor laser and an optical waveguide device. An optical waveguide device of the present invention is used as the optical waveguide device.
In the coherent light source of the present invention, it is preferable that the optical waveguide device is a second harmonic generation device and converts fundamental light having a wavelength of xcex1 that is emitted from the semiconductor laser into second harmonic light having a wavelength of xcex2. In this case, it is preferable that at least one of the entrance end face and the exit end face of the optical waveguide is provided with an antireflection film to be used for at least one of the fundamental light and the second harmonic light. Moreover, it is preferable that at least one of the entrance end face and the exit end face of the optical waveguide is provided with an antireflection film to be used for the fundamental light.
An optical apparatus according to the configuration of the present invention includes a coherent light source and a focusing optical system for focusing light emitted from the coherent light source on an object to be observed. A coherent light source of the present invention is used as the coherent light source. The optical waveguide device of the coherent light source and the object to be observed have a confocal relationship.
In the optical apparatus of the present invention, it is preferable that the object to be observed is an optical disk.